Ultrahigh-frequency transmission structure



Feb. 3, 1953 TAYLOR ETAL 2,627,551

ULTRAHIGH-FREQUENCY TRANSMISSION STRUCTURE Eiled March 2'5. 1950 Inv tors: Harr Ta glor, Alan J. Watts,

Their Aci-or'ne g.

Patented Feb. 3, 1953 ULTRAHIGH-FREQUENCY TRANSMIS SION STRUCTURE Harry B. Taylor, Ferry Hill, and Alan J. Watts,

Cambridge; England, assignors to General Electrio Company, a corporation of New York Application March 23, 1950, Serial No. 151,436 In Great Britain December 15, 1948 LClaim.

Our invention relates to structures for transferring energy at ultra-high-frequencies, and is particularly concerned with structures fortransferring energy between coaxialtransmission lines and rectangular wave guides.

One well known form of wave-guide-to-coaxial transmission line" transformer comprises a coaxial transmission linehaving its. end abutting the end of a rectangular waveguide. The outer sheath of the transmission line is electrically and mechanically joined, to' the outer walls of the wave guide and the inner conductor of the transmission lineextends slightly beyond the junction into the open end of the wave guide. In this known form of transformer, the inner conductor is bent in a smooth curve and terminated in a transverse conductive connection to the face of one wider inner wall of thewave guide. However, a transformer of: this type has been found to possess a resonant characteristic which is frequently a considerable disadvantagein. systems required to transfer a relatively wide band of frequencies, or in systems in. which there may be a drift in frequency.

It is a main object of our invention to provide an improved wave-guide-to-coaxial line transformer which overcomes this disadvantage and possesses a broad-band frequency characteristic.

Another object of our invention. is to provide an improved ultra-high-frec uency transmission structure which provides for efficient transfer of energy between a coaxial transmission line and rectangular wave guide with a minimum of attenuation and reflection of energy.

The principles of our invention also have particular application to an ultra-high-frequency transmission structure for effecting transfer of energy between two relativelyrotatable, rectangular wave guides- As will appear more fully from the following specification. this is accomplished through a combination of transformer. structures which provide a first transformation from one wave guide to a coaxial transmission line section and a second transformation from a second coaxial transmission line section to the other wave guide.

It is therefore a further object of our invention to provide an improved ultra-high-frequency rotating joint between two rectangular wave guides.

For additional objects and advantages and for a better understanding of the invention, attention is now directed to the following description and accompanying drawings. The features of the invention believed to be novel are particularly pointed out in the appended claim.

In the drawings:

Fig. 1 is a longitudinal elevational view, in cross-section, of a wave-guide-to-coaxial line transformer embodying our invention;

Fig. 2 is an end view looking toward the left in Fig. 1;

Figs. 3 and 4 are detail side and bottom views of the central conductor of the coaxial line section of Fig. 1 showing the manner in which it is constructed to provide the desired broad-band characteristic Fig. 5 is a sectional side elevation view through a rotatable joint constructed in accordance with our invention;

Fig. 6 is a vertical section view. on the line 6-,-6 of Fig. 5 looking toward the right; and

Fig. '7 is an end view looking toward the left in Fig. 5.

The structure of Fig. 1 provides a transformation between rectangular wave guide I and a coaxial transmission line having an outer sheath 2 and an inner coaxial conductor 3. The transformation occurs near the plane A-A through the end of the rectangular wave guide I. In order to provide good electrical contact between the walls of wave guide I and sheath 2, sheath 2 is preferably extended beyond the plane A--A and provided with a rectangular socket which has a close circumferential fit with the outer walls of wave guide I. The two are securely bonded together in any suitable manner, as by brazing or soldering. As best seen in Fig. 2, the sheath 2 preferably has a relatively thick cross-section and defines an inner cylindrical aperture 5 whose diameter is equal to the narrower transverse dimension inside the wave guide I.

The inner conductor 3 of the coaxial line extends a short distance into the open end of the wave guide and is provided with a bent or rounded end section 6 which makes good electrical contact with the center of the inner face of one wider wall of the wave guide l. In order to. insure good electrical and mechanical contact with the wall of wave guide 1, the end of the terminating portion 6 may be provided with a reduced cylindrical pin portion 1 which is seated in a hole drilled in the Wall of wave guide I and securely affixed thereto in any suitable manner, as by grazing or soldering.

It will be observed that the end portion 6 has non-parallel sides which flare outwardly as the wall of the wave guide is approached. As best shown in Fig. 3, the portion 6 is in the shape of a frustum of a. cone, the small end of which is formed into a smooth curve at the end of the inner conductor 3. For a band of wave-lengths of about 3.18 to 3.2 cms., it has been found that most efficient broadband transfer of energy is achieved when thecone has an apical angle of substantially 30 degrees. As shown in Fig. 3, one edge of the cone is at substantially 90 degrees'to the axis of the inner conductor 3. In other words, the axis of the cone is inclined at an obtuse angle of substantially 105 degrees to the axis of conductor 3. In a particular construction for operation over the above band of wave-lengths, the Wave guide had internal dimensions of inch by 'V8 inch and the coaxial line had an inner conductor inch in diameter located within a sheath hav' ing an internal diameter of it; inch.

Fig. 5 shows the application of our invention to a joint for permitting relative rotation-between two rectangular wave guides H and I2. Although there are numerous modes in which the electromagnetic 'energy may be propagated through the guides, the electromagnetic and electrostatic field distributions are never circularly symmetrical, as is well known. It is therefore not possible to couple the two wave guides together directly for relative rotation. In accordance with our invention, a transformation is effected from each wave guide to a section of coaxial cable in the manner previously described in connection with Figs. 1-4. Since these transmission line sections are circularly symmetrical, it is then possible to couple them for relative rotation in a manner to be described.

The left-hand section of the rotatable joint comprises the coaxial transmission line section having outer sheath l3 and inner conductor M. The sheath [3 is bonded to the outer walls of the wave guide II in the same manner as previously described for the elements 2, l of Fig. 1. The inner conductor 14 is provided with an end termination I5 corresponding to the termination 6 of Fig. 1. The wave guide I2 is similarly ter-'- minated in a coaxial transmission line section having outer sheath l6, inner conductor I7, and end termination 18.

In order to provide for transfer of electrical energy between the two transmission line sections in Fig. 5, without objectionable reflections or loss of energy, the abutting ends of the sections are inter-fitted in such a manner as to permit free rotational movement bout their common axis while providing electrical choke joints therebetween. For this purpose the inner conductor [4 is' provided with an end section !9 of reduced diameter'which extends into a cylindrical socket of slightly larger diameter in the end of conductor IT. The reduced portion !9 ha a length substantially equal to one-quarter of a wavelength at the mean operating frequency of the system and the socket or bore 20 has an electrical length substantially equal to one-half wavelength at this frequency. These elements together form a quarter-wave choke to provide efiicient transfer of energy between the conductors and minimum dissipation losses through the joint therebetween.

A similar quarter-wave choke is provided between the two sheaths I3 and 16. As shown, the sheath I3 is provided with a cylindrical aperture 2! into which is fitted a cylindrical sleeve portion 22 of the sheath l6. The total length of the recess between the two sheaths is substantially equal to one-half wave-length at the mean operating frequency, while the free end ofthe sleeve I6 projects into the recess 2| by a distance substantially one-quarter wave-length at this frequency.

The sleeves l3, l6 may be suitably supported in bearings (not shown) so that relative rotation between the two wave guides is permitted. At the same time transmission of ultra-high-frequency energy is effected without undue reflections or leakage as a result of the joint construction just described.

While certain specificembodiments have been shown and described, it will, of course, be understood by those skilled in the art that various modifications may be made without departing from the principles of our invention. The ppended claim is therefore intended to cover any such modification within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

A wave-guide-to-coaxial line transformer structure comprising a coaxial transmission line having an outer sheath and an inner conductor, a rectangular wave guide, said line and guide having abutting ends aligned on a common axis, means forming a continuous circumferential junction between said sheath and the walls of said guide, said inner conductor extending into the end of said guide, and a frusto-conical member forming an electrical connection from said inner conductor to the center of the inner face of one wider wall of said guide, said member having its axis inclined at substantially degrees to the axis of said conductor and having its sides subtending an apical angle of substantially 30 degrees.

. HARRY B. TAYLOR.

ALAN J. WATTS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Microwave Transmission Circuits, vol. 9, McGraw-Hill Book 00., N. Y., 1948; pages 449-450.

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